The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side, or through various primary-only feedback techniques.
Primary-only feedback techniques use the reflected voltage on the primary side of the transformer in each switching cycle. In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off. The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques use this reflected voltage to efficiently modulate the power switching and thus modulate the output voltage without requiring an optocoupler or other means to sense the output voltage.
As devices and chargers become more advanced, there is a need to transmit data signals from the secondary side of the transformer to the primary side. In that regard, it is conventional for a secondary-side controller to interface through a Universal Serial Bus (USB) cable to a mobile device. Not only does the USB cable provide power from the flyback converter but it also carries data between the secondary-side controller and the mobile device. Data signals can be used to provide device information and/or desired operational parameters, such as a desired output voltage, current, or other operational setting and/or protection settings, such as device component or charger component temperature limits, for example. It will be appreciated that the specific types of data being transferred is not limited to those described above and any type of useful information may be transferred using the techniques disclosed herein.
As explained above, the secondary side of a flyback power converter is isolated from the primary side for protection from the AC mains. Thus, data transfer from the secondary-side controller to the primary-side controller must also be similarly isolated. To provide this isolation, data signals may transferred using opto-isolators, or alternatively across the transformer for a more cost-effective design. One such method of transferring data across the transformer instead of an opto-isolator is to transmit a data bit after the transformer reset time and prior to the power transistor being placed in the ON state. This may be accomplished in a variety of ways. For example, the secondary-side controller can short out the secondary-side diode subsequent to the transformer reset time to cause a pulse of voltage to be reflected onto the primary winding. This pulse occurs while the voltage on the primary winding is resonantly oscillating. To provide a data signal with such a pulse, the secondary-side controller may select which cycle of the resonant oscillation in which to transmit the pulse. The primary-side controller may then identify the binary value of the transmitted pulse based upon in which resonant cycle the pulse is received. A similar pulse may be transmitted in synchronous rectification embodiments in which the secondary-side diode is replaced by a synchronous rectification switch transistor by closing the switch The secondary-side controller may be configured to cause a sequence of such voltage pulses across the secondary side winding of the switching power converter representing a digital message encoding one or more control parameters.
Using this method, one or more bits of data are transferred during each switching cycle. It will be appreciated that there are various techniques for indicating a bit state through such shorting of the rectification for the secondary winding. For example, the secondary communication controller may modulate the phase of the secondary resonance ringing to transmit data. The modulation may also comprise amplitude modulation. But regardless of how the data is transmitted through a momentary ceasing of the rectification on the secondary side winding current, such techniques are dependent on the power switching rate. In particular, note that the data transfer occurs during the off time following a power switch on period. The data transfer rate thus remains limited by the switching frequency of the power switch. Thus, while data transfer rates may be sufficient at relatively high switching frequencies, during low frequency operation (for example during low output load), the data rate may fall below a useful level, particularly for urgent messages such as an overvoltage message or an undervoltage message.
Thus, low load operation can cause significant problems for achieving useful data transfer rates in flyback converters. Moreover, note that merely increasing the switching frequency risks driving the output voltage and current out of regulation. Accordingly, there is a need in the art for an improved system and method of increasing the data transfer rate from the secondary-side controller to the primary-side controller of a flyback converter during low load levels.
In contrast to data communication through the transformer, an optocoupler provides the secondary-side controller the ability to send data at constant rate regardless of the power switching frequency. In that regard, the optocoupler would not be used to provide feedback information as that information would come through primary-only feedback methods. Instead, the optocoupler may be dedicated to data communication from the secondary-side controller to the primary-side controller. Although such communication is independent from the switching frequency, note that the primary-side controller has no knowledge when a given bit of a message is received as to whether such a bit is part of an urgent message or not. Should the message be urgent such as on over or under voltage message, the primary-side controller needs feedback information but may be in a low frequency mode such that current feedback information is unavailable. There is thus a need in the art for flyback converters using primary-only feedback techniques that may respond more quickly to urgent messages transmitted through an optocoupler.